This invention relates to electrostatographic processors having development and/or cleaning systems which utilize electrically conductive carrier particles and, more particularly, to means for reducing the degrading effect of carrier caused short circuits on the performance of such systems.
In a conventional electrostatographic printing process of the type described in Carlson's U.S. Pat. No. 2,297,691 on "Electrophotography", a uniformly charged imaging surface is selectively discharged in an image configuration to provide a latent electrostatic image which is then developed through the application of a finely divided, coloring material, called "toner". As is known, that process may be carried out in either a transfer mode or a non-transfer mode. In the non-transfer mode, the imaging surface serves as the ultimate support for the printed image. In contrast, the transfer mode involves the additional steps of transferring the developed or toned image to a suitable substrate, such as a plain paper, and then preparing the imaging surface for re-use by removing any residual toner particles still adhering thereto.
The Carlson patent specifically relates to xerography, which is probably the best example of the outstanding commercial success of the foregoing process in view of the widespread use of xerographic copiers and duplicators. Xerography, of course, involves the use of a photoreceptor as the imaging surface. Thus, it should be understood that there are other types of electrostatographic processors. For example, there are processors wherein the imaging surface is a uniformly charged insulator which is selectively discharged non-photographically -- e.g., by appropriately controlled stylii -- to provide a latent electrostatic image which permits of subsequent processing in essentially the same manner as the photographically generated latent image of a xerographic processor. Moreover, it should be noted that xerographic and similar electrostatographic printing processes are not limited to use in stand alone copiers and duplicators. For instance, those processes have also been found to have utility in the facsimile art.
One of the preferred vehicles for delivering the toner needed for development purposes is a multi-component developer comprising a mixture of toner particles and larger, so-called "carrier" particles. Normally, advantage is taken of a triboelectric charging process to induce electrical charges of opposite polarities onto the toner and carrier particles. To that end, the materials for the toner and carrier (or, sometimes, carrier coating) components of the developer are customarily selected so that they are removed from each other in the triboelectric series. Furthermore, in making those selections, consideration is given to the relative triboelectric ranking of the materials in order to ensure that the polarity of the charge nominally imparted to the toner particles opposes the polarity of the latent images of interest. Consequently, in operation, there are competing electrostatic forces acting on the toner particles of such a developer. Specifically, there are forces which tend to at least initially attract the toner particles to the carrier particles. Additionally, the toner particles are subject to being electrostatically stripped from the carrier particles whenever they are brought into the immediate proximity of or actual contact with an imaging surface bearing a latent image.
It has also been found that toner starved carrier particles (i.e., carrier particles which are substantially free of toner) may be employed in cleaning systems to remove residual or other weakly adhering toner particles from an imaging surface. To enhance that type of cleaning, provision is desirably made for treating the unwanted toner particles with a pre-cleaning corona discharge which at least partially neutralizes the forces holding them on the imaging surface, and then the carrier particles are brought into contact with the imaging suface to collect the toner particles.
Basically, the imaging surface of a xerographic or similar electrostatographic processor is an electrically insulating member which is deposited on an electrically conductive backing. Frequently, the development and cleaning systems of such processors include one or more electrodes so that electrostatic fields which improve the performance of those systems may be locally generated by holding the backing for the imaging surface at one potential while biasing the electrode or electrodes to a different potential. For example, development systems commonly include a development electrode to gain improved solid area coverage, and the development electrode is usually biased to suppress background development.
Heretofore, problems have been encountered in attempting to use electrically conductive carrier particles in systems relying on locally generated electrostatic fields. In particular, experience has demonstrated that conductive carrier particles occassionally cause short circuits which are transitory (typically, having a duration of less than about 50 microseconds), but nevertheless troublesome inasmuch as they upset the fields. Proposals have been made to alleviate some of the problems, but the art is still seeking a complete solution. For example, it has been suggested that the development electrode and housing of a development system should be maintained at the same potential, thereby preventing any current flow therebetween even should conductive carrier particles bridge the intervening space. However, that suggestion does not solve the problem which arises when there is a pin hole or other defect in the insulating imaging surface which permits a bridge-like accumulation of carrier particles to establish a short circuit between the electrode and the conductive backing for the imaging surface.
Understandably, therefore, electrically conductive carrier particles are not generally favored. That is unfortunate because conductive materials, such as bare nickel and iron beads, are sometimes the best possible choice for the carrier component. Specifically, there is evidence indicating that electrically conductive carrier particles would not only prolong the useful life of some developer mixtures, but also reduce the background development levels and the edge deletions caused by certain development systems.